1. Field of the Invention
The present invention relates to a lithographic projection apparatus and a device manufacturing method.
2. Description of the Related Art
The term xe2x80x9cpatterning devicexe2x80x9d as here employed should be broadly interpreted as referring to device that can be used to endow an incoming radiation beam with a patterned cross-section, corresponding to a pattern that is to be created in a target portion of the substrate. The term xe2x80x9clight valvexe2x80x9d can also be used in this context. Generally, the pattern will correspond to a particular functional layer in a device being created in the target portion, such as an integrated circuit or other device (see below). An example of such a patterning device is a mask. The concept of a mask is well known in lithography, and it includes mask types such as binary, alternating phase-shift, and attenuated phase-shift, as well as various hybrid mask types. Placement of such a mask in the radiation beam causes selective transmission (in the case of a transmissive mask) or reflection (in the case of a reflective mask) of the radiation impinging on the mask, according to the pattern on the mask. In the case of a mask, the support structure will generally be a mask table, which ensures that the mask can be held at a desired position in the incoming radiation beam, and that it can be moved relative to the beam if so desired.
Another example of a patterning device is a programmable mirror array. One example of such an array is a matrix-addressable surface having a viscoelastic control layer and a reflective surface. The basic principle behind such an apparatus is that, for example, addressed areas of the reflective surface reflect incident light as diffracted light, whereas unaddressed areas reflect incident light as undiffracted light. Using an appropriate filter, the undiffracted light can be filtered out of the reflected beam, leaving only the diffracted light behind. In this manner, the beam becomes patterned according to the addressing pattern of the matrix-addressable surface. An alternative embodiment of a programmable mirror array employs a matrix arrangement of tiny mirrors, each of which can be individually tilted about an axis by applying a suitable localized electric field, or by employing piezoelectric actuators. Once again, the mirrors are matrix-addressable, such that addressed mirrors will reflect an incoming radiation beam in a different direction to unaddressed mirrors. In this manner, the reflected beam is patterned according to the addressing pattern of the matrix-addressable mirrors. The required matrix addressing can be performed using suitable electronics. In both of the situations described hereabove, the patterning device can comprise one or more programmable mirror arrays. More information on mirror arrays as here referred to can be seen, for example, from U.S. Pat. Nos. 5,296,891 and 5,523,193, and PCT publications WO 98/38597 and WO 98/33096. In the case of a programmable mirror array, the support structure may be embodied as a frame or table, for example, which may be fixed or movable as required.
Another example of a patterning device is a programmable LCD array. An example of such a construction is given in U.S. Pat. No. 5,229,872. As above, the support structure in this case may be embodied as a frame or table, for example, which may be fixed or movable as required.
For purposes of simplicity, the rest of this text may, at certain locations, specifically direct itself to examples involving a mask and mask table. However, the general principles discussed in such instances should be seen in the broader context of the patterning device as hereabove set forth.
Lithographic projection apparatus can be used, for example, in the manufacture of integrated circuits (IC""s). In such a case, the patterning device may generate a circuit pattern corresponding to an individual layer of the IC, and this pattern can be imaged onto a target portion (e.g. comprising one or more dies) on a substrate (silicon wafer) that has been coated with a layer of radiation-sensitive material (resist). In general, a single wafer will contain a whole network of adjacent target portions that are successively irradiated via the projection system, one at a time. In current apparatus, employing patterning by a mask on a mask table, a distinction can be made between two different types of machine. In one type of lithographic projection apparatus, each target portion is irradiated by exposing the entire mask pattern onto the target portion at once. Such an apparatus is commonly referred to as a wafer stepper. In an alternative apparatus, commonly referred to as a step-and-scan apparatus, each target portion is irradiated by progressively scanning the mask pattern under the projection beam in a given reference direction (the xe2x80x9cscanningxe2x80x9d direction) while synchronously scanning the substrate table parallel or anti-parallel to this direction. Since, in general, the projection system will have a magnification factor M (generally less than 1), the speed V at which the substrate table is scanned will be a factor M times that at which the mask table is scanned. More information with regard to lithographic devices as here described can be seen, for example, from U.S. Pat. No. 6,046,792.
In a known manufacturing process using a lithographic projection apparatus, a pattern (e.g. in a mask) is imaged onto a substrate that is at least partially covered by a layer of radiation-sensitive material (resist). Prior to this imaging, the substrate may undergo various procedures, such as priming, resist coating and a soft bake. After exposure, the substrate may be subjected to other procedures, such as a post-exposure bake (PEB), development, a hard bake and measurement/inspection of the imaged features. This array of procedures is used as a basis to pattern an individual layer of a device, e.g. an IC. Such a patterned layer may then undergo various processes such as etching, ion-implantation (doping), metallization, oxidation, chemo-mechanical polishing, etc., all intended to finish off an individual layer. If several layers are required, then the whole procedure, or a variant thereof, will have to be repeated for each new layer. It is important to ensure that the overlay (juxtaposition) of the various stacked layers is as accurate as possible. For this purpose, a small reference mark is provided at one or more positions on the wafer, thus defining the origin of a coordinate system on the wafer. Using optical and electronic devices in combination with the substrate holder positioning device (referred to hereinafter as xe2x80x9calignment systemxe2x80x9d), this mark can then be relocated each time a new layer has to be juxtaposed on an existing layer, and can be used as an alignment reference. Eventually, an array of devices will be present on the substrate (wafer). These devices are then separated from one another by a technique such as dicing or sawing, whence the individual devices can be mounted on a carrier, connected to pins, etc. Further information regarding such processes can be obtained, for example, from the book xe2x80x9cMicrochip Fabrication: A Practical Guide to Semiconductor Processingxe2x80x9d, Third Edition, by Peter van Zant, McGraw Hill Publishing Co., 1997, ISBN 0-07-067250-4.
For the sake of simplicity, the projection system may hereinafter be referred to as the xe2x80x9clens.xe2x80x9d However, this term should be broadly interpreted as encompassing various types of projection system, including refractive optics, reflective optics, and catadioptric systems, for example. The radiation system may also include components operating according to any of these design types for directing, shaping or controlling the projection beam of radiation, and such components may also be referred to below, collectively or singularly, as a xe2x80x9clensxe2x80x9d. Further, the lithographic apparatus may be of a type having two or more substrate tables (and/or two or more mask tables). In such xe2x80x9cmultiple stagexe2x80x9d devices the additional tables may be used in parallel or preparatory steps may be carried out on one or more tables while one or more other tables are being used for exposures. Dual stage lithographic apparatus are described, for example, in U.S. Pat. No. 5,969,441 and WO 98/40791.
For the sake of simplicity, the projection system may hereinafter be referred to as the xe2x80x9clensxe2x80x9d. However, this term should be broadly interpreted as encompassing various types of projection systems, including refractive optics, reflective optics, and catadioptric systems, for example. The radiation system may also include elements operating according to any of these design types for directing, shaping or controlling the projection beam of radiation, and such elements may also be referred to below, collectively or singularly, as a xe2x80x9clensxe2x80x9d or as a xe2x80x9cprojection system elementxe2x80x9d. The projection system of a lithographic apparatus such as that described above must project the patterned beam onto the target portion of the substrate with very high accuracy and without introducing errors such as optical aberration or displacement errors, for example, into the projected image. Refractive and/or reflective elements within the projection system may need to be accurately positioned in one or more (up to six) degrees of freedom per optical element (which may include, for example, linear displacement along three orthogonal axes and rotational displacement around the three axes). A projection system comprising reflective optics is, for instance, disclosed in U.S. Pat. Nos. 5,815,310 and 5,956,192.
The accuracy of the positioning of such projection system elements may be affected by vibrations and other positional noise within the projection system and the projection system frame. The positional noise may, for example, be created by influences external to the projection system (such as acoustic noise, residual floor vibrations and scan reaction forces transmitted via the vibration isolation or suspension system between the base frame and the projection optics system) and by internal influences such as the reaction forces of the actuators used to adjust the position of the projection system elements. The projection system elements may be mounted directly to a common frame of the projection system or may be mounted on a sub-frame that is mounted to the common frame. Further, the above considerations may also apply to the radiation system and projection system elements mounted therein.
It is an aspect of the present invention to reduce the effect of any disturbing forces and/or displacements within the projection or radiation system on the respective projection or radiation system elements mounted therein.
This and other objects are achieved according to the invention in a lithographic apparatus including a radiation system constructed and arranged to supply a projection beam of radiation; a support structure constructed and arranged to support a patterning device, the patterning device constructed and arranged to pattern the projection beam according to a desired pattern; a substrate table constructed and arranged to hold a substrate; and a projection system constructed and arranged to project the patterned beam onto a target portion of the substrate, wherein at least one of the projection and radiation systems comprises at least one optical element connected by an actuator to a reaction mass that is moveable in at least one degree of freedom, the force exerted by the actuator between the reaction mass and the optical element being used to control the position of the optical element in one or more degrees of freedom.
The present invention can thereby enable sub-nanometer position control of the transmissive and reflective optical elements in the optical (projection or radiation) system. This is done by a position control of one or more (up to six) degrees of freedom of the relative position of the optical element to the frame of the optical system. The greatly improved positional accuracy for the optical elements in the projection system or radiation system provides a corresponding improvement in imaging quality.
In an exemplary embodiment of the present invention, the reaction mass is compliantly mounted to a frame of the apparatus, such as a frame of the projection system, and the reaction to the force exerted by the actuator between the reaction mass and the optical element is used to move the optical element to the required position. This can prevent transmission of vibrations caused by the actuator when adjusting the position of the optical element to the remainder of the apparatus and thence to other optical elements. In addition it can prevent vibrations in the projection system or the radiation system, for example vibrations caused by sources external to the projection or radiation system from being transferred to the optical element.
In a further exemplary embodiment of the present invention, the reaction mass is connected only to the optical element and an accelerometer is used to detect the movement of the optical element. Using data from the accelerometer, the reaction to the force exerted by the actuator between the reaction mass and the optical element can be used to reduce the movement of the optical element. This can reduce any residual vibration in the optical element.
According to a further aspect of the invention there is provided a device manufacturing method including providing a substrate that is at least partially covered by a layer of radiation-sensitive material; providing a projection beam of radiation using a radiation system; using a patterning device to endow the projection beam with a pattern in its cross-section; projecting the patterned beam of radiation onto a target portion of the layer of radiation-sensitive material using a projection system, wherein an actuator is connected between a reaction mass that is moveable in at least one degree of freedom and an optical element in at least one of the projection and radiation systems; and using the force exerted by the actuator between the reaction mass and the optical element to control the position of the optical element.
Although specific reference may be made in this text to the use of the apparatus according to the invention in the manufacture of IC""s, it should be explicitly understood that such an apparatus has many other possible applications. For example, it may be employed in the manufacture of integrated optical systems, guidance and detection patterns for magnetic domain memories, liquid-crystal display panels, thin-film magnetic heads, etc. The skilled artisan will appreciate that, in the context of such alternative applications, any use of the terms xe2x80x9creticlexe2x80x9d, xe2x80x9cwaferxe2x80x9d or xe2x80x9cdiexe2x80x9d in this text should be considered as being replaced by the more general terms xe2x80x9cmaskxe2x80x9d, xe2x80x9csubstratexe2x80x9d and xe2x80x9ctarget portionxe2x80x9d, respectively.
In the present document, the terms xe2x80x9cradiationxe2x80x9d and xe2x80x9cbeamxe2x80x9d are used to encompass all types of electromagnetic radiation, including ultraviolet radiation (e.g. with a wavelength of 365, 248, 193, 157 or 126 nm) and extreme ultra-violet (EUV) radiation (e.g. having a wavelength in the range 5-20 nm), as well as particle beams, such as ion beams or electron beams.